Field of the Disclosure
The disclosure relates to a clock synchronization method and a mobile network system, a network controller, and a network switch.
Description of Related Art
Along with the continued development of fourth generation (4G) long term evolution (LTE) technology, base station (BS) clock synchronization is a required feature due to the technical requirement of LTE time division duplex (LTE-TDD) and the technologies of the enhanced Inter-cell Interference Coordination (eICIC) in third generation partnership project (3GPP) Release-10. However, the clock synchronization technology of Backhaul Network is becoming more important because of the increase in the number of BSs and the difficulty for obtaining the provisioning locations of GPS antennas. The techniques of clock synchronization in a network may be, for example, Network Timing Protocol (NTP), Synchronous Ethernet and IEEE 1588v2 Precision Time Protocol (PTP). Wherein, the IEEE 1588v2 PTP provides frequency and time synchronization at the same time and has a sub-microsecond or higher accuracy, therefore the IEEE 1588v2 PTP is currently the most important clock synchronization technology in mobile network.
Precision Time Protocol (PTP), as defined in the IEEE 1588v2 (i.e., IEEE-1588-2008) standard entitled “Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems,” enables precise synchronization of clocks over a data packet network. In a nutshell, PTP is used to distribute a “grandmaster” clock's precise time-of-day to slave clocks. This is done using an exchange of PTP packets, which include timestamps carried inside. Slave clocks adjust these timestamps to account for end-to-end delay, and can obtain a local time-of-day (TOD) aligned to the grandmaster with sub-microsecond precision, in some cases.
While PTP messages are generally sent using multicast messaging, IEEE 1588v2 includes specifications to allow a master clock to negotiate unicast transmission on a port-by-port basis. PTP messages used by ordinary and boundary clocks include Sync, Delay_Req, Follow_Up and Delay_Resp messages, which are used to transfer time-related information across the network. Additional message types are used by so-called “transparent clocks,” to measure delays across the network for improved correction of transferred TOD. These include Pdelay_Req and Pdelay_Resp messages.
The clock synchronization technology of the IEEE 1588v2 PTP is a master-slave protocol. In the process of switching multiple packets between a master device and a slave device, the slave device can calculate and obtain the transmission delay time (Delay) and the time offset (Offset) from the master device, then correct the clock of the slave device to synchronize with the master device. However, the accuracy of the IEEE 1588v2 PTP depends on two factors. The first factor is the symmetry of the network path, wherein when the routing path from the master device to the slave device and the routing path from the slave device to the master device are asymmetry, calculating the delay time will cause an error. The second factor is the delay variation of packets transmission (that is, the transmission delay time is unstable). The unstable transmission delay time will cause difficult computations of the time offset, therefore the accuracy of synchronization protocol will be reduced.
FIG. 1A is a schematic diagram illustrating a clock synchronization architecture in an existing mobile network. Referring to FIG. 1A, the clock synchronization architecture in the existing mobile network, an operator sets a Grandmaster clock as a master clock in the entire network. The master clock transmits the synchronous message from the network switch to the BS in a stepwise fashion by a boundary clock (BC) and a transparent clock (TC), thus the BS synchronizes with the Grandmaster clock. However, operators in actually deploying the mobile network may face two challenges of clock synchronization issues.
FIG. 1B is a schematic diagram illustrating a clock synchronization architecture may face the challenges in the existing mobile network. Referring to FIG. 1B, the clock synchronization of deploying the mobile network, the first challenge of the clock synchronization issues is that the backhaul network may have excessive network switches for transmitting a synchronous message, or the backhaul network may exist third-party untrusted networks. In this case, it may cause the significant delay variation for transmitting the clock synchronous message because of factors such as the clock synchronous message transmission may not be guaranteed or the third-party untrusted networks may not support an IEEE 1588v2 PTP. Therefore, it will affect the synchronous operation between the Grandmaster clock and the BS. In addition, the second challenge of the clock synchronization issues is, when the well-established mobile network and the Grandmaster clock cannot synchronize with the BS, operators are difficult to detect this condition which is caused by the network switch. The operators must spend more manpower and time to detect the network switches one by one to find out which network switch breaks down, thus resulting in a substantial decrease in the service quality and increasing operating costs.